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I wanted to know the hosts that were up on my network. So while working with Nmap I couldn't understand why using the command nmap -sn 192.168.2.1/150 it didn't want to tell me the hosts that were up, But then after using nmap -sn 192.168.2.*, it worked for me instead of the other one. Can somebody explain me this event?

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    Why did you use 192.168.2.1/150? What were you thinking that would do?
    – schroeder
    Dec 30, 2017 at 20:44
  • 1
    Maybe they meant the range 192.168.2.1-150.
    – Ben S
    Dec 30, 2017 at 22:35
  • I was using 192.168.2.1/150 because the devices of my network are above 192.168.2.100 :)
    – VladiC4T
    Dec 31, 2017 at 20:47

2 Answers 2

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Your first example uses the CIDR notation to specify the host range as a netmask.

But since /150 is an illegal value for an IPv4 subnet mask, nmap falls back to the largest plausible value which is /32 (because an IPv4 address only has 32 bits), making you scan exactly one host:

$ nmap -sn 192.168.2.1/150

Starting Nmap 7.60 ( https://nmap.org ) at 2017-12-30 19:45 CET
Illegal netmask in "192.168.2.1/150". Assuming /32 (one host)
Nmap scan report for easy.box.local (192.168.2.1)
Host is up (0.0026s latency).
Nmap done: 1 IP address (1 host up) scanned in 0.01 seconds

The CIDR-noted range equivalent to

$ nmap -sn "192.168.2.*"

is

$ nmap -sn 192.168.2.1/24

because 255.255.255.0 has 24 leading 1 bits.

You can verify that assumption with more verbosity (-v):

$ nmap -sn -v 192.168.2.1/24

Starting Nmap 7.60 ( https://nmap.org ) at 2017-12-30 19:58 CET
Initiating Ping Scan at 19:58
Scanning 256 hosts [2 ports/host]
Completed Ping Scan at 19:58, 2.40s elapsed (256 total hosts)
Initiating Parallel DNS resolution of 256 hosts. at 19:58
Completed Parallel DNS resolution of 256 hosts. at 19:58, 0.06s elapsed
Nmap scan report for 192.168.2.0 [host down]
Nmap scan report for 192.168.2.1 [host down]
...
Nmap scan report for 192.168.2.255 [host down]
Read data files from: /usr/bin/../share/nmap
Nmap done: 256 IP addresses (2 hosts up) scanned in 2.47 seconds
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The specified network to scan in this command nmap -sn 192.168.2.1/150 is invalid.

You requested prefix 150, but it is invalid.

If you want to scan 192.168.2.*, you should use prefix 24.

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